I promise you -- you have. It's an interesting phenomenon, since the statement is one of agreement or affirmation, yet it begins with a term of disagreement or negation. Why, you may ask? I have asked as well. Habits of speech? I would say so.

In any case, now that you've heard about it, you can start listening for it. You may be surprised at how often you hear it -- I was (especially from myself).

You may think this topic has no place here, but my experience has been, since I noticed this, that when I hear it, I find it rather amusing. Hence, the location. (Everything but this paragraph cross-posted at geekiness...)